Correction of wafer temperature drift in a plasma reactor based upon continuous wafer temperature measurements using an in-situ wafer temperature optical probe

ABSTRACT

The invention solves the problem of continuously monitoring wafer temperature during processing using an optical or fluoro-optical temperature sensor including an optical fiber having an end next to and facing the backside of the wafer. This optical fiber is accommodated without disturbing plasma processing by providing in one of the wafer lift pins an axial void through which the optical fiber passes. The end of the fiber facing the wafer backside is coincident with the end of the hollow lift pin. The other end is coupled via an “external” optical fiber to temperature probe electronics external of the reactor chamber. The invention uses direct wafer temperature measurements with a test wafer to establish a data base of wafer temperature behavior as a function of coolant pressure and a data base of wafer temperature behavior as a function of wafer support or “puck” temperature. These data bases are then employed during processing of a production wafer to control coolant pressure in such a manner as to minimize wafer temperature deviation from the desired temperature.

RELATED APPLCIATIONS

[0001] This application is a divisional of U.S. application Ser. No.09/547,359, filed Apr. 11, 2000.

BACKGROUND OF THE INVENTION

[0002] Precise control of plasma processing of semiconductor wafersrequires that the wafer temperature be carefully regulated or held veryclose to a desired temperature. Drift of wafer temperature causesvarious characteristics of the plasma process to change, so that theprocess cannot be accurately controlled. For example, in a plasma etchprocess, the etch rate may increase if the wafer temperature driftstoward a higher temperature. Typically, plasma processing will increasethe wafer temperature. In order to maintain the wafer temperature at adesired level, the wafer is cooled during plasma processing by pumping acoolant such as Helium gas through the wafer support through coolantpassages which permit the coolant to conduct heat away from the waferbackside. The cooling rate is proportional to the pressure at which thecoolant is supplied to the coolant passages in the wafer support, sothat the wafer temperature is directly affected by the coolant pressure.Thus, it is conventional to set the coolant pressure to a constant valuecorresponding to a desired wafer temperature during processing.

[0003] Heretofore, it has been assumed that with the foregoing coolingtechnique, wafer temperature is generally constant during plasmaprocessing of the wafer. This is an important assumption, particularlyin the case of nitride processes, because plasma etch processes foretching nitride layers are very sensitive to changes in temperature.Wafer temperature has typically been measured using conventional(commercially available) “temperature dots” which can be stuck to thesurface of a test wafer. At the conclusion of plasma processing of thetest wafer, the color of the temperature dot indicates the highesttemperature reached during the process. Assuming the wafer temperatureis constant over the duration of the plasma process, this temperature isgenerally taken to be the process temperature of the wafer.

[0004] However, it has not been practical to test the assumption ofconstant wafer temperature during the process. For example, at plasmaignition, it is assumed the wafer temperature climbs very quickly (e.g.,within a matter of seconds) from room temperature to the steady-stateplasma processing temperature. This assumption could not be verifiedbecause direct measurement of wafer temperature during processing—andparticularly during processing of production wafers—has not beenpractical. The only temperature that can be continuously monitoredduring processing is the wafer support or “puck” temperature, which isat a temperature significantly lower than that of the wafer duringprocessing.

[0005] As described below, one aspect of the present invention providesa highly accurate probe with which the temperature of a test waferhaving a special dye in a predetermined spot can be continuouslymonitored during plasma processing. With this probe, it has beendiscovered that the wafer temperature is not constant during processing,because, among other things, the wafer takes a surprisingly long time(over one minute) to climb from room temperature to steady state processtemperature following plasma ignition. Such variations in wafertemperature are detrimental because they tend to reduce the precisionwith which the process parameters (e.g., etch rate) may be controlled.It was also discovered that, in a plasma reactor employing anelectrostatic chuck, the wafer temperature climbs very high near the endof plasma processing. This is because coolant pressure is removed beforeturning off RF power to the electrostatic chuck holding the wafer on thewafer support. Otherwise, the wafer would be blown off the wafer supportby the coolant pressure on the wafer backside as soon as RF power isremoved from the electrostatic chuck.

[0006] Thus, it is a discovery of the invention that there is a need tosense deviations in wafer temperature from a desired temperature and tosomehow correct such deviations. While this is certainly possible in thecase of a test wafer whose temperature throughout processing iscontinuously monitored using the probe of the invention referred toabove, it does not seem possible in the case of a production wafer whichshould not be contaminated with the dye required for the probe tomeasure wafer temperature.

[0007] Thus, there is a need for a way of deducing in real timedeviations of the temperature of a production wafer from a desiredtemperature during processing, without being able to directly measurethe wafer temperature. Further, there is a need for a way of changingthe system in response to such deviations to minimize or avoid them.

SUMMARY OF THE INVENTION

[0008] The invention solves the problem of continuously monitoring wafertemperature during processing using an optical or fluoro-opticaltemperature sensor including an optical fiber having an end next to andfacing the backside of the wafer. This optical fiber is accommodatedwithout disturbing plasma processing by providing in one of the waferlift pins an axial void through which the optical fiber passes. The endof the fiber facing the wafer backside is coincident with the end of thehollow lift pin. The other end is coupled via an “external” opticalfiber to temperature probe electronics external of the reactor chamber.In a preferred embodiment, the hollow lift pin is supported with theother lift pins on a lift spider and a flexible bellows assembly. Theoptical fiber inside the hollow lift pin and the external optical fiberare preferably coupled together by a flexure near the bottom of thebellows. Preferably, a cavity smaller than the diameter of the opticalfiber is drilled in the wafer backside in registration with the opticalfiber inside the lift pin, and a suitable dye is deposited in the cavityto facilitate temperature sensing by the sensor.

[0009] The invention also solves the problem of determining wafertemperature deviations in production wafers in which there is no cavitynor dye in the wafer backside enabling temperature measurement by theoptical probe. The invention solves this problem by first, using directwafer temperature measurements with a test wafer, establishing a database of wafer temperature behavior as a function of coolant pressure andestablishing a data base of wafer temperature behavior as a function ofwafer support or “puck” temperature. These data bases are then employedduring processing of a production wafer to control coolant pressure insuch a manner as to minimize wafer temperature deviation from thedesired temperature.

[0010] In a preferred embodiment, such control of the coolant pressureis accomplished by first measuring puck temperature and using the database of wafer temperature as a function of puck temperature to deducethe wafer temperature from the measured puck temperature. Then, thewafer temperature thus deduced is compared with a desired temperature tocalculate an error. This error is used with the data base of wafertemperature as a function of coolant pressure to determine a change inthe coolant pressure which will tend to correct the error in the mannerof a feedback control system.

[0011] In another embodiment, the corrections to the coolant pressureare established in a trial-and-error method. Thus, for example, thedelay in wafer temperature rise immediately after plasma ignition iscorrected by a corresponding delay in applying or increasing the coolantpressure immediately after plasma ignition, the coolant pressure beinggradually increased in accordance with a schedule (of coolant pressureas a function of time after plasma ignition) that permits the wafertemperature to increase very quickly to the desired temperature andremain there. The trial-and-error method of establishing the schedule ofcoolant pressure is carried out with a test wafer using the opticalwafer temperature probe of the invention. The coolant schedule ismodified over successive attempts until a fairly constant wafertemperature from plasma ignition onward is achieved. Then, the scheduleof coolant pressure is applied to production wafers.

[0012] In a yet further embodiment, the wafer temperature behaviorobserved in test wafers with the optical probe of the invention isparameterized in an equation as a function of coolant pressure and pucktemperature. This equation is then employed to accurately calculatecoolant pressure corrections based upon continuously measured pucktemperature.

[0013] The problem of wafer temperature increase just prior to waferde-chuck from the electrostatic chuck (i.e., when the coolant pressureis turned off) is solved by reducing the RF power on the electrostaticchuck to a level at which heat transfer to the wafer is reduced butwhich is still sufficient to electrostatically retain the wafer on thechuck.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross-sectional side view of a portion of a plasmareactor including a preferred embodiment of the optical temperatureprobe of the invention.

[0015]FIG. 2 illustrates the behavior of wafer temperature at plasmaignition using constant coolant pressure on the wafer backside.

[0016]FIG. 3 illustrates a system, including a plasma reactor having theoptical temperature probe of FIG. 2, for characterizing wafertemperature behavior and for controlling coolant pressure to minimizewafer temperature drift.

[0017]FIG. 4 illustrates a method of operating the system of FIG. 3 tocharacterize or measure the temperature behavior of a test wafer as afunction of coolant pressure and as a function of wafer support or pucktemperature.

[0018]FIG. 5 illustrates a method of regulating the temperature of aproduction wafer by monitoring wafer support temperature or pucktemperature using the wafer temperature behavior characterized in themethod of FIG. 4.

[0019]FIG. 6 is a graph illustrating the behavior of wafer temperatureas a function of coolant pressure as measured with a test wafer usingthe optical probe of FIG. 1.

[0020]FIG. 7 is a graph illustrating the behavior of wafer temperatureas a function of puck temperature as measured with a test wafer usingthe optical probe of FIG. 1.

[0021]FIG. 8 is a graph illustrating how a fairly constant wafertemperature is achieved at plasma ignition by increasing the coolantpressure in a schedule of steps established by trial and error.

[0022]FIG. 9 is a graph corresponding to FIG. 8 in which the samereactor with the same process parameters was operated in theconventional manner with constant coolant pressure immediately uponplasma ignition, and illustrating for the sake of comparison the veryslow rise of the wafer temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Wafer Temperature Optical Probe

[0024] Referring to FIG. 1, a plasma reactor 100 has a vacuum chamberenclosure 102 including a ceiling 105 and cylindrical side wall 107adjoining a floor 109. An electrostatic chuck 111 on the floor 109includes a ceramic (aluminum nitride) puck 113 on a support 115 and anelectrode mesh 117 inside the ceramic puck 113. A semiconductor wafer119 is held firmly onto the ceramic puck 113 by an electrostatic forceinduced by a chucking voltage from a chuck voltage source 121 applied tothe electrode mesh 117. The ceramic puck 113 has coolant passages 123 inits top surface which permit a coolant circulated therethrough (such ashelium gas) to directly contact the backside of the wafer 119. For thispurpose, a coolant pump 125 pumps helium gas into the coolant passages123 at a selected gas pressure. The wafer 119 is lifted off theelectrostatic chuck 111 by plural lift pins 127 which extend throughoutthe electrostatic chuck 111 to contact the backside of the wafer 119.The lift pins 127 are moved up and down by a lift spider 129 through abellows assembly including a rod 131 underlying each pin 127, a portionof the rod being held within a flexible bellows 133. The rod 131supports a piston plate 135 and housing 137 to which the lift pin 127 isthreadably connected and sealed by an O-ring 138. The wafer 119 isplaced on the electrostatic chuck 111 by lowering the lift spider 129until the wafer backside touches the puck 113. Then, the chuckingvoltage source 121 is activated to apply a chucking voltage to theelectrode mesh 117. Thereafter, the coolant pump 125 pumps coolant intothe passages 123 to cool the wafer 119. A plasma is ignited byintroducing a processing gas through gas inlets 139 into the reactorchamber and coupling RF power into the chamber. The power may be coupledcapacitively by applying it to the electrode mesh 117 and grounding theceiling 105. Alternatively, the RF power may be coupled inductively byapplying RF power to a coil antenna 141 adjacent the wall 107 or ceiling105, in which case an RF bias voltage may be applied to the electrodemesh 117 to control ion energy near the wafer 119.

[0025] In order to obtain an indirect indication of the temperature ofthe wafer 119, a puck temperature probe 143 provides a continuous readout of the temperature of the puck 113 during plasma processing.Heretofore, it has not been practical to obtain a continuous directmeasurement of wafer temperature, notwithstanding the criticality of thewafer temperature during certain plasma processes such as a nitride etchoperation, for example.

[0026] In order to solve this problem, one of the cylindrical lift pins127 is made to be hollow, and a light pipe 145 is inserted through thehollow lift pin 127. In addition, the rod 131 is also hollow so that thelight pipe 145 extends through the interior of the rod 131 as well. Thetop end of the light pipe 145 coincides with the top of the lift pin 127against the wafer backside. The bottom end of the light pipe 145 extendsto the lift spider 129 and is coupled to a flexible optical fiber 147.For this purpose, the light pipe is fed through the top of the liftspider 129 by a flexure 149 and is terminated at the bottom of the liftspider 129 at one end of a standard fiber optic connector 151. Theflexible optical fiber 147 is connected to the other end of the fiberoptic connector 151. While the light pipe 145 may be a flexible quartzoptical fiber, it is preferably a sapphire rod because it has a highernumerical aperture than a quartz fiber. The opposite end of the flexibleoptical fiber 147 is connected to conventional probe electronics 155,which may be, for example, a Luxtron Fluoro-Optical Temperature (FOT)probe. This type of probe requires a chemical dye be placed on thesurface whose temperature is to be measured by the probe. Therefore, amechanical cavity 160 having a diameter less than that of the lift pin127 is By formed in the backside of the test wafer 119, the mechanicalcavity 160 being in registration with the lift pin 127 and light pipe145. The requisite chemical dye or phosphor 165 is placed in the cavity160. The presence of the light pipe 145 does not compromise the plasmaprocessing because the light pipe is isolated from the plasma eventhough it may contact or at least nearly contact the backside of thewafer 119. This is because the open end of hollow lift pin 127 abuts thewafer backside while the wafer backside is electrostatically clamped tothe puck 113. Moreover, the lift pin 127 is sealed to the housing 137 bythe O-ring 138. Therefore, the wafer temperature optical probe (i.e.,the light pipe 145, optical fiber 147 and FOT 155) of the inventionprovides a continuous measurement of temperature of the wafer 119.

[0027] Method for Controlling Wafer Temperature

[0028] The probe of FIG. 1 has enabled for the first time an accurateassessment of wafer temperature behavior throughout the entire plasmaprocess cycle. FIG. 2 is a graph illustrating the wafer temperaturemeasured with the probe of FIG. 1 over time beginning with plasmaignition at 0 seconds and concluding with wafer de-chucking at about 240seconds. What FIG. 2 indicates is that the assumptions regarding fairlyconstant wafer temperature behavior were incorrect, in that the wafertemperature does not reach the equilibrium plasma processing temperaturearound 90 degrees C for nearly two minutes. Thus, for the first twominutes of a plasma process cycle that is only four minutes in duration,the wafer temperature is not in control and is below the desiredtemperature, in the example of FIG. 2. FIG. 2 also shows that at 210seconds when the helium coolant pressure is removed in preparation fordechucking the wafer, the wafer temperature soars by about 15 degreesuntil RF power is turned off at about 235 seconds.

[0029] In order to solve the problem of the slow warming up of the waferduring the first 90 seconds after plasma ignition, the invention uses alarge data base gathered using the optical temperature probe of FIG. 1with a test wafer. One difficulty is that a straightforward approach ofdirectly monitoring the temperature of each wafer being processed usingthe probe of FIG. 1 and altering the coolant pressure in a directlycontrolled feedback control loop to maintain the desired wafertemperature is not possible except in the special case of a test waferhaving the mechanical cavity 160 and phosphor dye placed therein.Unfortunately, a production wafer cannot have such a cavity nor achemical dye or phosphor placed thereon. Therefore, direct continuoustemperature measurement of a production wafer is not possible, even withthe probe of FIG. 1. The closest temperature to the wafer 119 that canbe continuously monitored during processing of production wafers is thatof the puck 113. However, tests conducted using the wafer temperatureprobe of FIG. 1 and a temperature probe in the puck 113 reveal that thepuck temperature is well below the wafer temperature, particularlyduring the first 120 following plasma ignition. Therefore, the pucktemperature cannot be substituted for the wafer temperature in a controlloop, particularly during the early part of the plasma process cyclefollowing plasma ignition.

[0030] The foregoing problems are solved in the invention by firstemploying a test wafer with the probe of FIG. 1 and a puck temperatureprobe and correlating the temperature behavior of the wafer thetemperature behavior of the puck. Furthermore, the test wafer isemployed with the probe of FIG. 1 to correlate wafer temperature withthe coolant pressure. Then, whenever a production wafer is processed,the puck temperature is monitored, and the actual wafer temperature isinferred from the measured puck temperature using the correlation ofwafer and puck temperatures obtained with the test wafer. If theinferred temperature of the production wafer deviates from a desiredwafer temperature, a correction to the coolant pressure is inferred fromthe observed deviation using the correlation of wafer temperature andcoolant pressure obtained with the test wafer. The result is that theslow increase in wafer temperature is automatically corrected by aseries of coolant temperature corrections, in which the coolant pressureis suppressed near the beginning of the plasma process and is increasedduring the process to maintain the desired wafer temperature.

[0031] Alternatively, the optimum coolant pressure profile over time maybe found by trial and error using a test wafer and the probe of FIG. 1.The coolant pressure is controlled in accordance with this optimumprofile during the processing of production wafers.

[0032]FIG. 3 illustrates a system for carrying out the foregoingtemperature control methods. The plasma reactor of FIG. 1 is illustratedin FIG. 3 as including, in addition to the features discussed above withreference to FIG. 1, a puck temperature probe 305, a gas pressure probe310 and the wafer temperature probe of FIG. 1 designated by thereference numeral 315 in FIG. 3. The puck temperature probe 305 consistsof a conventional temperature sensor inside the puck 113. The heliumpressure probe 310 consists of a conventional pressure sensor inside oneof the coolant passages 123. Each of the three probes 305, 310, 315 hasits output connected to a microprocessor 320 which uses a memory 330.

[0033] Referring to FIG. 4, the test wafer 119 is chucked on theelectrostatic chuck 111 (block 410 of FIG. 4), a plasma is struck andthe wafer temperature and puck temperature are continuously monitoredand recorded by the microprocessor 320 (block 420 of FIG. 4). Themicroprocessor 320 (or a human) varies the coolant pressure controlledby the pump 125 (block 430 of FIG. 4). This variation may be performedas part of a feedback control loop in an effort to maintain wafertemperature at a desired level, or else it may be a trial and erroriterative process to find the best coolant pressure time profile. Theresulting data from the three probes 305, 310, 315 is stored by themicroprocessor 320 in the memory 330 (block 440). This data is thenorganized into a first look-up table (look-up table 1) correlatingsimultaneous readings from the wafer temperature probe 315 and thecoolant pressure probe 310 (block 450 of FIG. 4). The data is alsoorganized into a second look-up table (look-up table 2) correlatingsimultaneous readings from the wafer temperature probe 315 and the pucktemperature probe 310 (block 460 of FIG. 4). Look-up tables 1 and 2 arethen used to control helium (coolant) pressure based upon only measuringpuck temperature during the plasma processing of production wafers.

[0034] Referring to FIG. 5, a production wafer (i.e., a semiconductorwafer lacking the mechanical cavity 160 formed in the test wafer 119 ofFIG. 1) is chucked onto the electrostatic chuck 111 (block 510 of FIG.5). The production wafer preferably has no cavity 160 and no dye on itsbackside, in order to satisfy specifications for production wafer purityand structural integrity. The optical temperature probe (i.e., thehollow lift pin 127, the light pipe 145 and the FOT 155, etc.) cannot beused with the production wafer because of the absence of any optical dyeon the production wafer's backside. Therefore, one reactor, a testreactor including the optical probe, may be used to carry out the methodof FIG. 4, while another reactor, a production reactor used to carry outthe method of FIG. 5, may not necessarily include the opticaltemperature probe of FIG. 1. Alternatively, the same reactor thatincludes the optical probe may be used to carry out both operations.Continuing now with the description of the method of FIG. 5, a plasma isstruck and the puck temperature is continuously monitored during plasmaprocessing (block 515). Each sample of the puck temperature is used bythe computer 320 to infer a corresponding wafer temperature usinglook-up table 2. From this, a wafer temperature drift (e.g., fromdesired temperature) is inferred (block 520). The temperature drift isused to infer from look-up table 1 an optimal correction to the coolantpressure (block 525). The coolant pressure is then correctedaccordingly, and the process repeats itself with the next sample of pucktemperature (block 530).

[0035]FIG. 6 is a graph depicting the data corresponding to look-uptable 1 correlating simultaneous wafer temperature measurements andhelium pressure measurements at two different RF power levels, 1.8kWatts (upper curve) and 1.0 kWatts (lower curve). FIG. 7 is a graphdepicting the data corresponding to look-up table 2 correlatingsimultaneous measurements of wafer temperature and puck temperature. Inthe graphs of both FIG. 6 and FIG. 7, plasma ignition begins at time 0seconds. FIG. 7 shows that the difference between puck and wafertemperatures varies with the puck temperature. The behavior illustratedin FIG. 7 was obtained by varying the helium pressure in accordance withan optimal helium pressure time profile for maintaining a nearlyconstant wafer temperature (by permitting the wafer to reach itsequilibrium temperature very quickly after plasma ignition). Thisprofile was obtained by trial and error and is illustrated in FIG. 8. InFIG. 8, the step-wise curve is helium pressure as a function of time,and is the optimum profile referred to above. The upper curve in FIG. 8is the wafer temperature as a function of time. Referring again to FIG.7, the puck temperature apparently approaches the steady state wafertemperature in an asymptotic fashion and therefore the differencebetween wafer and puck temperature depends on the instantaneous pucktemperature. The microprocessor 520, in controlling the temperature of aproduction wafer, correlates the current puck temperature with theclosest puck temperature in look-up table 2 (whose contents correspondsto the graph of FIG. 7), and finds the corresponding wafer temperature.For example, a puck temperature of 50 degrees C in FIG. 7 corresponds toa wafer temperature of about 65 degrees C. If the desired wafertemperature were, for example, 60 degrees C, then an error of +5 degreesC has been detected. The correction to the coolant pressure is obtainedby referring to the data of look-up table 1 represented by the graph ofFIG. 6. Specifically, assuming the RF power is 1.8 kWatts, an excursionfrom 65 degrees to 60 degrees C corresponds to an increase in heliumpressure of 1 Torr. Therefore, the helium pressure correction to beapplied in this example is an increase of 1 Torr. This method is only afirst order approximation but provides an improvement over conventionaltechniques which have generally assumed no need for wafer temperaturecorrection.

[0036] Yet another approach is to use curve-fit the data of FIGS. 6 and7 (look-up tables 1 and 2) to approximate constants of a quadraticequation expressing the wafer temperature T as a function of heliumpressure P and puck temperature E:

T=AP ² +BP+CE.

[0037] Once the constants A, B and C are found, the variation in heliumpressure P to correct T is found in straight-forward fashion usingconventional mathematical techniques carried out by the microprocessor520.

[0038] Referring to FIG. 8, the problem of wafer temperature rise justprior to wafer de-chuck, when the helium pressure is removed, is solvedby briefly reducing the RF power applied to the plasma to about 100Watts.

[0039]FIG. 9 illustrates for the sake of comparison the inferior resultsobtained in the same reactor under the same conditions when the heliumpressure is held constant throughout the process in accordance withconventional techniques. For nearly half of the duration of the process,the wafer temperature is below the steady state operating temperature.

[0040] While the invention has been described in detail by specificreference to preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

What is claimed is:
 1. In a plasma reactor having a vacuum chambercontaining a wafer support for supporting a semiconductor wafer to beprocessed and plural elongate lift pins extending through said wafersupport to abut the backside of said wafer at the top ends of said liftpins, a temperature measurement system for continuously monitoring thetemperature of said wafer during plasma processing of said wafer in saidreactor, said system comprising: a light pipe extending through one ofsaid lift pins, and having a top end generally coincident with the topend of said one lift pin, said light pipe having a bottom end extendingthrough the bottom end of said hollow lift pin; and an opticaltemperature measuring sensor coupled to the bottom end of said lightpipe.
 2. The temperature measurement system of claim 1 wherein saidlight pipe comprises one of: a sapphire rod; a quartz optical fiber. 3.The temperature measurement system of claim 1 further comprising aflexible optical fiber having one end connected to said bottom end ofsaid light pipe and its other end connected to said optical temperaturemeasuring sensor.
 4. The temperature measurement system of claim 3wherein said reactor further comprises a lift spider supporting thebottom ends of said lift pins, wherein said flexible optical fiberflexes to accommodate movement of said lift spider and lift pins.